The invention relates to a method of producing mirrors, and to coated glass substrates incorporating highly reflecting "mirror" coatings.
The light reflecting properties of mirrors are generally provided by a layer of highly relecting metal, especially silver, aluminium or chromium, applied to a glass or plastics substrate; copper layers are sometimes used as an alternative, but are generally less acceptable because of the strong red tint of the reflected light.
Silver coatings are generally applied to preformed glass plates, in the cold, by wet chemical methods in which a solution of silver salt is applied to the glass surface and reacted with a reducing agent which reduces silver ions present to silver metal which deposits on the glass surface. The silver used is not very durable in use and in practice requires protection by other layers, and these methods are generally unsuitable for application to glass on the production line on which it is formed so that a separate "silvering" line is required to produce the silvered glass.
Aluminium coatings are difficult to apply by chemical methods because of the strongly reducing nature of aluminium metal, and aluminium mirrors are generally produced by deposition methods carried out at low pressure e.g. by sputtering. Such low pressure methods are essentially batch processes and, like the wet chemical methods used for deposition of silver mirrors, are generally unsuitable for on-line application on the production line on which the glass is made.
GB 2248853A discloses a method of coating glass with aluminium to form a mirror. A solution of an alane amine adduct of aluminium is formed and the liquid is deposited onto heated glass. The adduct decomposes to form an aluminium coating. Although it is stated that it is envisaged that the invention may be used in conjunction with float glass production, there is no exemplification of such a use. It is believed that substantial technical problems could be encountered in simply introducing the disclosed aluminium compounds into a float glass line.
Silicon layers have also been used to produce reflecting layers (which, like silver and aluminium layers, are substantially neutral in reflection colour) on architectural glazing for aesthetic and solar control purposes. GB 1507465, 1507996 and 1573154 relate to a continuous chemical vapour deposition method for producing float glass having such a silicon layer, and U.S. Pat. No. 4,661,381 describes a development of that method. However, such silicon layers do not provide the high reflections commonly required in mirrors. Thus REFLECTAFLOAT (trade mark) glass, commercially available from Pilkington Glass Limited of St. Helens, England, has a reflection of about 50%, and MIRROPANE EP (trade mark) commercially available from Libbey-Owens-Ford Co. has a reflection of about 60%.
None of the above technology is currently suitable for the application of highly reflecting coatings to glass during the glass production process to provide a coated glass substrate with a light reflection of over 70%, and preferably over 80%.
On a completely different scale, it has been proposed in GB 1262163, to produce very highly reflecting (greater than 90%) "cold light" mirrors comprising silicon layers for use, for example in cinema projectors, for separating heat radiation from visible light. Such cold light mirrors are produced by vacuum deposition on thin bases, typically glass substrates 3 mm thick or less, and are used without any backing paint to minimise build up of heat in the glass. GB 1262163 refers, in discussing the prior art, to a known cold light mirror comprising a "purest silicon layer" covered by four to six alternate layers of silicon oxide and tantalum oxide or titanium oxide but concludes that, for a satisfactory product, substantially more layers would be required. It therefore proposes to achieve the very high reflection (greater than 90%) required in a different way using several silicon layers as the individual layers of high refractive index of a multi-layer interference system.
Much more recently, it has been proposed by J. Stone and L. W. Stulz (Applied Optics, February 1990, Volume 29, No. 4) to use quarter wavelength stacks of silicon and silica layers for mirrors in the spectral region between 1.0 and 1.6 microns (i.e. within the infra red). However, the authors observe that silicon cannot be used at wavelengths below about 1 micron (and thus not in the visible region of the spectrum) due to its high absorption at such wavelengths. Stone and Stulz refer to the deposition of Si/SiO.sub.2 by low pressure methods such as reactive sputtering and electron beam evaporation.
Although GB 1262163 and the Stone and Stulz paper are discussed herein, the technology, in particular the production process described therein, is not suitable for the production of on line glass mirrors which essentially requires processes suitable for use at atmospheric pressure. Accordingly, these references would not be considered by the person skilled in the art as being in any way relevant to the production of on-line mirrors to compete with the conventional "off-line" mirrors discussed above.